Mechanical mixers for mixing components to homogeneity are well known. Their applications include, but are not limited to baking, building construction and medicine.
Mixing apparatus for high viscosity mixtures are typically adapted to provide sufficient shear force to continue moving against great resistance. In some cases, the resistance increases during mixing because the viscosity of the mixture increases.
One example of a case where the viscosity of the mixture increases during mixing is preparation of a polymer/monomer mixture. When a polymer and monomer are combined, a polymerization reaction begins. The polymerization reaction increases the average polymer chain length in the mixture and/or causes cross-linking between polymer chains. Increased polymer chain length and/or cross linking between polymer chains contribute to increased viscosity.
Polymerization mixtures are often employed in formulation of bone cement. One common polymer/monomer pair employed in bone cement formulation is polymethylmethacrylate/methylmethacrylate (PMMA/MMA). Because PMMA/MMA bone cements typically set to a solid form, reaction conditions for the polymerization reaction are generally adjusted so that mixing PMMA and MMA produces a liquid phase which lasts several minutes. This is typically achieved by mixing a monomer liquid including MMA and, optionally DMPT and/or HQ, with a polymer powder including PMMA and, optionally Barium Sulfate and/or BPO and/or styrene. Typically, known mixing apparatuses are constructed for use with a liquid polymerization mixture and may not be suitable for mixing of highly viscous cements that have substantially no liquid phase during mixing.
One problem that is typically encountered with some prior art systems derives from the delivery and transfer of the liquid and powder components of the bone cements into the mixing apparatus. These components must be kept separate from each other until the user is ready to mix them. Typically, the dry powder is stored in a flexible bag, while the liquid monomer is stored for shipment and handling in a vial or an ampoule, usually formed from glass; both require opening and pouring into a mixing well prior to mixing. Typically the liquid monomer has a foul odor.
U.S. Pat. No. 6,572,256 to Seaton et al, the disclosure of which is fully incorporated herein by reference, describes a fluid transfer assembly detachably coupled to a mixing vessel. The assembly is designed to dispense a liquid monomer component from a sealed unit in a closed loop operation. The closed-loop operation is facilitated by a vacuum source connected to the mixing vessel though a portal and used as a driving force to suck liquid out of the sealed unit once pierced by a hollow needle.